When using a charge pump to divide an input supply to a lower voltage it is desirable to use as few external components (i.e., external to an integrated circuit or IC) in an efficient manner. A step-down charge pump 100 (for example and as shown in FIG. 1) can be used because it has a small number of modes or switching schemes in which it can operate. Generally, this charge pump 100 generally comprises a driver 102, a charge pump circuit 104, and storage capacitors CVDD and CVSS. In operation, charge pump 100 uses three phases 202, 204, and 206 to generate the step down voltages (which can be seen in FIGS. 2A through 2C). In the first phase 202, the driver 102 couples flying capacitors C1 to CN (which are included in the charge pump circuit 104) in series between an input terminal VIN (which supplies an input or supply voltage) and an output terminal CPVDD (which supplies a positive output voltage) so as to charge the flying capacitors C1 to CN. The second and third phases 204 and 206 (which are alternated with the first phase 202) generate positive and negative output voltages at terminals CPVDD and CPVSS, respectively, by coupling the charged flying capacitors C1 to CN in parallel to the storage capacitors CVDD and CVSS, respectively. The largest divide ratio (input voltage divided by output voltage) with the charge pump 100 is N+1, so with two flying capacitors (for example) the largest or maximum divide ratio is 3. Thus, there has generally been a tradeoff between the number of flying capacitors and the divide ratio.
Lower voltages than available with conventional charge pumps alone can be generated with the combination of a step-down charge pump (i.e., charge pump 100) and an LDO (Low Drop-out Regulator). However, this is an inefficient solution as power is wasted in the LDO. Thus, there is a need for an improved charge pump.
An examples of conventional circuits is Texas Instruments' TPS60500 and TPA2055D3.